It has been common in the past to use a single network of sewers to convey sewage and storm runoff to a central point for disposal. The combination of storm and sanitary sewers floods the sewage treatment plant every time it rains, and the sewage is not adequately treated, storm water is not carried off at an adequate rate, or both problems occur. Thus, the art has recently felt a need for means to prevent storm runoff from entering sanitary sewer systems.
One entry point for storm water into a sanitary sewer is through the manholes which provide access to the sanitary sewer; it is important to seal these manholes. The art has realized this need to the extent of providing a seal between the cover and cover frame of a manhole, and by providing manhole covers which are impervious to storm water. But water can also enter the manhole through the corbel joint between the casing and cover frame of the manhole, either by seeping along the outside of the cover frame from the surface or by flowing as subsurface water under the pavement and through the corbel joint.
This joint is difficult to seal. First, iron and masonry parts interface at the corbel joint, and each has a different thermal coefficient of expansion, causing relative movement between the cover frame and casing at the corbel joint as the ambient temperature changes. Second, the casing moves relative to the cover frame each time a heavy vehicle passes near or over the manhole. Finally, the manhole cover frame frequently needs to be lifted when a road is repaved or when the manhole cover frame has settled into an existing pavement. When the manhole is raised, the cover frame and manhole casing can become separated, or an additional joint is formed between them. The prior art has not addressed the problem of sealing a manhole corbel joint.
Another problem with some manhole sealing means known to the prior art is the need to replace existing manhole covers, the cover frames, or other parts of a manhole to seal the manhole against entry of water. Since many areas already have a great number of manholes, the need to replace parts of a manhole assembly before they wear out or are broken can exact a great financial burden on the government responsible for sealing the manhole.
Finally, some prior art manhole sealing means raise the manhole cover above pavement level, causing damage to the manhole when vehicles strike the raised manhole cover, and making the real surface uneven.